The invention relates to a method for estimating a frequency error in a communication system, in particular in a mobile communication system, in which method sample sets are successively sampled from a signal to be received at specific points corresponding to the symbol sequence points of a receiver and frequency error estimates that correspond to each sampling point are determined on the basis of the sample sets sampled at said sampling points.
When establishing a connection between two communication devices a receiving device has to become synchronized with a signal to be received in order that transmission of information would be possible. Particularly in mobile communication systems, in which intercommunicating devices have no separate synchronizing connection, but each device comprises an oscillator of its own, the frequencies of the devices need to be mutually synchronized when a connection is being established. For instance, in TETRA (Terrestrial Trunked Radio) system, when operating at a frequency of 400 MHz, a typical maximum deviation of a base station is xc2x140 Hz from the nominal frequency and the corresponding value of a terminal is xc2x1800 Hz. If two terminals communicate with each other (a so-called direct-mode channel), the mutual frequency difference of the devices may at worst rise up to 1600 Hz. When the base station and the terminal communicate with each other, the difference may even in that case exceed 800 Hz. However, for the connection to work, the difference can be only about 300 Hz at most. When establishing a connection between two communication devices, first, the frequency error of a signal to be received as compared with the receiver frequency has to be determined and the frequency of the receiving device has to be tuned accordingly.
There are several known methods for detecting and compensating the frequency error. For instance, the receiver can be tuned to different frequencies until the correct frequency has been found. It is also known to sample the signal to be received and to estimate the signal frequency error on the basis of the samples, for instance, by means of a Fourier transformation or by using a MUSIC (Multiple Signal Classification) algorithm.
A drawback with these known methods is slowness. Traditionally, in cellular communications systems there is ample time to determine and compensate the frequency error of the signal to be received. Instead, in the new TETRA system, for instance, the call establishment is carried out considerably quicker than in previous systems and this sets stricter requirements for the time available for determining the frequency error. Particularly quick error determination is required when operating in direct-mode channel of the TETRA system. If the frequency error of the signal to be received is not determined and compensated in time, it may result in a total loss of incoming information.
The object of this invention is to provide a method by means of which a frequency error of a signal to be received can be determined quickly with a sufficient accuracy, in order that a receiver could be tuned to a correct frequency prior to the actual transmission. This is achieved with the method of the invention, the method being characterized in that the variance of frequency error estimates of each sampling point is calculated and the frequency error estimate of the signal to be received is determined on the basis of the frequency error estimates of the sampling point with the least variance.
The method of the invention is based on the idea that variance is used for selecting the most reliable frequency error estimate from a plurality of estimates calculated on the basis of sample sets sampled from the signal. An advantage with the method is its quickness as compared with the previously known methods. In the TETRA system, a frequency error up to 2 kHz can be corrected with the method of the invention sufficiently quickly.